The demand for information transmission bandwidth has been explosively growing in recent years. To meet needs of increasing network traffic, 40 Gbps and 100 Gbps optical communication networks have been deployed commercially in backbone networks, and 400 Gbps and 1 Tbps optical communication systems are being studied. On the access network side, higher requirements are also proposed for network traffic and multi-service support capabilities. At present, the access network generally employs a passive optical network (PON) with a tree structure, especially a time division multiplexing passive optical network (TDM-PON). Ethernet passive optical network (EPON) and gigabit-capable passive optical network (GPON) technologies are mostly used in the current fiber to the home (FTTH) network construction. However, they cannot meet the demand of the access network for higher data rate any longer. As such, the next generation of PON (NG-PON) technology has drawn much attention in the industry. It is generally believed in the industry that evolution of the NG-PON technology involves the following three aspects: 1. increasing data rate of a single wavelength; 2. wavelength division multiplexing; and 3. orthogonal frequency division multiplexing.
Each of the three solutions can effectively solve the problem of bandwidth bottleneck in the future, but they also have their own problems to be solved. For example, the first solution of increasing data rate of the single wavelength will inevitably lead to greater dispersion, and the third solution of orthogonal frequency division multiplexing puts forward new requirements on digital signal processing (DSP). In contrast, the second solution of wavelength division multiplexing is relatively easy to implement, with lower technical barriers, and relatively lower cost. For this reason, the full service access networks (FSAN) summit specified time and wavelength division multiplexed passive optical network (TWDM-PON) as the ultimate solution for the next generation of PON products in April 2012. However, even TWDM-PON also has its own technical problems to be solved. For example, an optical network unit (ONU) module must have wavelength-adjustable transmitting and receiving functions. That is, both the optical wavelength of the transmitting section and the optical wavelength of the receiving section have to be adjustable. In other words, the ONU module needs to integrate a bi-directional optical sub-assembly (BOSA) having an adjustable wavelength in both directions.
Currently, the wavelength-adjustable transmitter and the wavelength-adjustable receiver, as commonly used devices in optical communication, have been intensively studied. However, they often have a simple function and cannot adjust the wavelength at the transmitting side and the receiving side concurrently, so they cannot meet requirements of the TWDM-PON network.
PCT Patent Application No. WO2005036239A2 entitled “TUNABLE FILTER MEMBRANE STRUCTURES AND METHOD OF MAKING” discloses a filter medium membrane formed by deposition, and it adjusts wavelength by heating multiple films. This approach is hard to implement and greatly depends on the manufacture apparatus so that it is not easy for mass production. In addition, it cannot realize wavelength adjustment in two directions and thus cannot meet the requirements of the TWDM-PON network.
Chinese Patent Application No. 201410794542.0 entitled “ADJUSTABLE OPTICAL RECEIVER AND ITS ADJUSTABLE FILTER USED IN TWDM-PON SYSTEM” discloses a F-P cavity filtering etalon assembly formed by combining two or more F-P cavity filtering etalons having the same free space spectrum width, and temperature of the F-P cavity etalons may be controlled to adjust length of the F-P cavity and thus the filtering wavelength. This solution has advantages such as a simple structure, easy controlling, and good filtering effect, but it cannot realize optical signal transmitting and can only be used as a receiver. In this regard, it has a simple function and cannot meet the requirements of the TWDM-PON network.
There are many simple optical signal transmitter devices in the market, such as distributed feedback lasers (DFBs) and electro-absorption modulated lasers (EMLs), which are of various types and have different performance. For a particular device, however, it has a limited range of optical transmitting power adjustment, and can only be used as a transmitter. They have a simple function and cannot meet the requirements of the TWDM-PON network.